The immune system, particularly cytotoxic T lymphocytes (which, hereinafter, may be abbreviated to CTLs) play an important role in the exclusion of cancer in vivo. Infiltration of cytotoxic T lymphocytes exhibiting a cytotoxic activity against tumor cells has been detected at the tumor site of a cancer patient (Arch. Surg., 126:200–205, 1990.) A target molecule (tumor antigen) of the tumor-specific cytotoxic T lymphocytes was first discovered in a melanoma. A tumor antigen generated in a tumor cell is degraded in the cell into a peptide (tumor antigen peptide) consisting of 8 to 11 amino acids, which binds to a human leukocyte antigen (HLA) molecule that is the major histocompatibility complex antigen to be displayed on the surface of the tumor cell. The cytotoxic T lymphocytes recognize a complex consisting of HLA and the tumor antigen peptide, and damage the tumor cell. In other words, the cytotoxic T lymphocytes recognize the tumor antigen peptide in an HLA-restricted manner.
HLA is a cell membrane antigen, and is expressed on almost all eukaryotic cells. HLA is mainly classified into class I antigen and class II antigen. The HLA recognized by the cytotoxic T lymphocytes together with an antigen peptide belongs to class I antigens. HLA class I antigens are further classified into HLA-A, HLA-B, HLA-C, and so on. It was reported that HLA has genetic polymorphism. The HLA-A2 allele, which is one of polymorphisms of HLA-A subregion, is found in approximately 23% of African Blacks, approximately 53% of Chinese, approximately 40% of Japanese, approximately 49% of Northern Caucasians, and approximately 38% of Southern Caucasians.
As used herein, a tumor antigen means a protein, a polypeptide, or a peptide, which constitutes part of the tumor cell and is capable of inducing tumor-specific cytotoxic T lymphocytes. A tumor antigen peptide means a peptide that is generated as a result of degradation of the tumor antigen in a tumor cell and can induce or activate tumor-specific cytotoxic T lymphocytes upon being expressed on the cell surface by binding to an HLA molecule. In addition, the site of the amino acid sequence which is capable of inducing tumor-specific cytotoxic T lymphocytes that is present in a tumor antigen is called a tumor antigen epitope (tumor antigen determinant.)
In recent years, many genes encoding tumor antigens that can be recognized by cytotoxic T lymphocytes have been identified from cDNA of human tumor cells (Science 254:1643–1647, 1991; J. Exp. Med. 183:1185–1192, 1996; J. Immunol. 163:4994–5004, 1999.) Some of these genes are involved in cellular proliferation and malignant transformation, including HER/neu (Proc. Natl Acad. Sci. USA, 92:432–436, 1995) mutant cdk (Science, 269:1281–1284, 1995) mutant CASP-8 (J. Exp. Med., 186:785–793, 1997) and so on.
On the other hand, a molecule such as a tumor rejection antigen gene and a T cell antigen receptor (TCR), which are involved in specific immunity, have been identified in melanoma, esophageal cancer, and other cancers in the past 10 years, and a specific immunotherapy of advanced cancer or metastatic cancer has been studied using the peptide.
Now, in Europe and in the United States, cancer vaccine therapy has been developed in which cytotoxic T lymphocytes are activated by an administration of a tumor antigen in a cancer patient. Results from a clinical test of a melanoma specific tumor antigen have been reported. For example, administration of a melanoma antigen gp-100 peptide subcutaneously to melanoma patients along with administering interleukin-2(IL-2) intravenously gave a tumor regression in 42% of the patients (Nature Medicine, 4:321, 1998.) In this way, by utilizing a tumor antigen as a vaccine, an effective treatment against cancer can be achieved.
However, almost all of the identified tumor antigens are derived from melanomas. Tumor antigens derived from epithelial cancers and adenocarcinomas, such as pancreatic cancer, which occur at high incidence rates, have been reported for such specific immunotherapy only in a few papers. Pancreatic cancer is one of the largest causes of death by cancer in the world and causes about 27,000 deaths a year in the USA and about 50,000 deaths in Europe. The main factors causing these large numbers of deaths are lack of an effective therapeutic method, the difficulty of diagnosis, and the activity of this cancer. Only 1 to 4% of pancreatic cancer patients have overcome the disease, and the incidence substantially equals the death rate. Therefore, a new approach of therapy, for example, development of specific immunotherapy is needed.
In addition, in view of the diversity of cancer, an identical tumor antigen should not be expressed in the same degree in all cancer cells. Naturally, cancer vaccine therapy by activating the cytotoxic T lymphocytes using one kind of tumor antigen has a therapeutic effect on cancer having the tumor antigen. However, in order to induce and activate the tumor antigen-specific cytotoxic T lymphocytes and obtain a high therapeutic effect corresponding to the diversity of cancer, it is important to discover and use many novel tumor antigens in accordance with the diversity of cancer.